1,6-Hexanediol is useful as a starting material for producing polyurethane, polyester-type plasticizers, unsaturated polyesters, 1,6-hexanediol diacrylate, etc. (see JP-A-62-184640, JP-A-56-78844, JP-A-3-227389, and JP-A-5-59306, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
It is known that 1,6-hexanediol is prepared by a process comprising oxidizing cyclohexane to form adipic acid, .epsilon.-hydroxycaproic acid, etc., esterifying these carboxylic acids with methanol, etc., and reacting the resulting esters with hydrogen in the presence of a catalyst for hydrogenation (see JP-B-53-33567 (the term "JP-B" as used herein means an "examined published Japanese patent application") and JP-A-51-108040). This process is disadvantageous in that an esterification step is required and that the hydrogenation should be conducted in the presence of a copper catalyst under high temperature and high pressure conditions.
There have been proposed several processes in which 1,6-hexanediol is obtained through direct hydrogenation of the starting compound without involving esterification. For example, JP-A-47-4767 and JP-A-49-132003 disclose hydrogenation in the presence of a cobalt catalyst. JP-B-49-33171 and JP-A-9-5918.8 propose using a rhenium catalyst and a Raney nickel catalyst, respectively. However, these processes have their several problems such that the reaction conditions are severe or the catalyst activity is unsatisfactory.
Further, JP-A-47-4764 and JP-A-48-14609 teach a process comprising catalytically hydrogenating a starting carboxylic acid in the form of an alkali metal salt in a buffer solution. This process is disadvantageous in that an alkali equivalent to the carboxyl group of the starting compound is required and that the hydrogenation should be catalytically carried out in a buffer solution by using a cobalt catalyst under high pressure.